BGP Configuration
slides from Michael Schapira and Jen Rexford
Wyatt Lloyd
Fall 2012 (TTh 1:30-2:50 in COS 402)
COS 561: Advanced Computer Networks
http://www.cs.princeton.edu/courses/archive/fall12/cos561/
Local Control vs. Global Properties
The Internet is a “network of networks”
–~35,000 separately administered networks
–Competitive cooperation for e2e reachability
Local Control
Intradomain routing,
interdomain policies
Global Properties
Performance, security,
reliability, scalability
2
Two-Tiered Routing Architecture
• Goal: distributed management of resources
–Internetworking of multiple networks
–Networks under separate administrative control
• Intradomain: inside a region of control
–Routers configured to achieve a common goal
–Okay for routers to share topology information
–Different ASes can run different protocols
• Interdomain: between regions of control
–ASes have different (maybe conflicting) goals
–Routers only share reachability information
3
Internet Structure
4
Autonomous Systems (ASes)
• AS-level topology
–Nodes are Autonomous Systems (ASes)
–Destinations are prefixes (e.g., 12.0.0.0/8)
–Edges are links and business relationships
4
3
5
2
1
Client
7
6
Web server
5
AS Numbers (ASNs)
ASNs are 16 bit values (or 32-bit).
64512 through 65535 are “private”
Currently around 35,000 in use.
•
•
•
•
•
•
•
•
•
Level 3: 1
MIT: 3
Harvard: 11
Yale: 29
Princeton: 88
AT&T: 7018, 6341, 5074, …
Verizon: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
…
6
Business Relationships Between ASes
• Neighboring ASes have business contracts
–How much traffic to carry
–Which destinations to reach
–How much money to pay
• Common business relationships
–Customer-provider
–Peer-peer
–Backup
–Sibling
7
Customer-Provider Relationship
• Customer needs to be reachable from everyone
– Provider ensures all neighbors can reach the customer
• Customer does not want to provide transit service
– Customer does not let its providers send traffic through it
Traffic to the customer
Traffic from the customer
d
provider
provider
traffic
customer
d
customer
8
Peer-Peer Relationship
• Peers exchange traffic between customers
– AS lets its peer reach (only) its customers
– AS can reach its peer’s customers
– Often the relationship is settlement-free (i.e., no $$$)
Traffic to/from the peer and its customers
peer
d
traffic
peer
9
AS Structure: Tier-1 Providers
• Top of the Internet hierarchy
–Has no upstream provider of its own
–Typically has a large (inter)national backbone
–Around 10-12 ASes: AT&T, Sprint, Level 3, …
peer-peer
peer-peer
peer-peer
peer-peer
10
AS Structure: Other ASes
• Lower-layer providers (tier-2, …)
–Provide transit service to downstream customers
 But need at least one provider of their own
–Typically have national or regional scope
 E.g., Minnesota Regional Network
–Includes a few thousand ASes
• Stub ASes
–Do not provide transit service
–Connect to upstream provider(s)
–Most ASes (e.g., 85-90%)
–E.g., Princeton
11
Policy-Based Path-Vector Routing
12
Shortest-Path Routing is Restrictive
• All traffic must travel on shortest paths
• All nodes need common notion of link costs
• Incompatible with commercial relationships
National
ISP1
Regional
ISP3
Cust3
National
ISP2
Regional
ISP2
Cust2
YES
NO
Regional
ISP1
Cust1
13
Path-Vector Routing
• Extension of distance-vector routing
–Support flexible routing policies
–Faster convergence(avoid count-to-infinity)
• Key idea: advertise the entire path
–Distance vector: send distance metric per dest d
–Path vector: send the entire path for each dest d
“d: path (2,1)”
3
“d: path (1)”
1
2
data traffic
data traffic
d
14
Faster Loop Detection
• Node can easily detect a loop
– Look for its own node identifier in the path
– E.g., node 1 sees itself in the path “3, 2, 1”
• Node can simply discard paths with loops
– E.g., node 1 simply discards the advertisement
“d: path (2,1)”
3
“d: path (1)”
2
“d: path (3,2,1)”
1
15
Flexible Policies
• Each node can apply local policies
– Path selection: Which path to use?
– Path export: Whether to advertise the path?
• Examples
– Node 2 may prefer the path “2, 3, 1” over “2, 1”
– Node 1 may not let node 3 hear the path “1, 2”
2
3
1
2
3
1
16
Border Gateway Protocol
Border Gateway Protocol
• Prefix-based path-vector protocol
• Policy-based routing based on AS Paths
• Evolved during the past 20 years
• 1989 : BGP-1 [RFC 1105], replacement for EGP
• 1990 : BGP-2 [RFC 1163]
• 1991 : BGP-3 [RFC 1267]
• 1995 : BGP-4 [RFC 1771], support for CIDR
• 2006 : BGP-4 [RFC 4271], update
“BGP at 18”: http://www.youtube.com/watch?v=HAOVNYSnL7k
18
BGP Operations
Establish session on
TCP port 179
AS1
BGP session
Exchange all
active routes
AS2
Exchange incremental
updates
While connection
is ALIVE exchange
route UPDATE messages
19
Incremental Protocol
• A node learns multiple paths to destination
–Stores all of the routes in a routing table
–Applies policy to select a single active route
–… and may advertise the route to its neighbors
• Incremental updates
–Announcement
 Upon selecting a new active route, add node id to path
 … and (optionally) advertise to each neighbor
–Withdrawal
 If the active route is no longer available
 … send a withdrawal message to the neighbors
20
BGP Route
• Destination prefix (e.g., 128.112.0.0/16)
• Route attributes, including
– AS path (e.g., “7018 88”)
– Next-hop IP address (e.g., 12.127.0.121)
192.0.2.1
AS 7018
12.127.0.121
AT&T
AS 88
AS 11
Yale
Princeton
128.112.0.0/16
AS path = 88
Next Hop = 192.0.2.1
128.112.0.0/16
AS path = 7018 88
Next Hop = 12.127.0.121
21
BGP Path Selection
• Simplest case
AS 1129
–Shortest AS path
–Arbitrary tie break
• Example
Global Access
128.112.0.0/16
AS Path = 1129 1755 1239 7018 88
–Three-hop AS path preferred
over a five-hop AS path
–AS 12654 prefers path
through Global Crossing
• But, BGP is not limited to
shortest-path routing
–Policy-based routing
AS 12654
RIPE NCC
RIS project
128.112.0.0/16
AS Path = 3549 7018 88
AS 3549
Global Crossing
22
BGP Policy: Influencing Decisions
Open ended programming.
Constrained only by vendor configuration language
Receive Apply Policy =
filter routes &
BGP
Updates tweak attributes
Apply Import
Policies
Based on
Attribute
Values
Best
Routes
Best Route
Selection
Best Route
Table
Apply Policy =
filter routes &
tweak attributes
Transmit
BGP
Updates
Apply Export
Policies
Install forwarding
Entries for best
Routes.
IP Forwarding Table
23
BGP Policy: Applying Policy to Routes
• Import policy
–Filter unwanted routes from neighbor
 E.g. prefix that your customer doesn’t own
–Manipulate attributes to influence path selection
 E.g., assign local preference to favored routes
• Export policy
–Filter routes you don’t want to tell your neighbor
 E.g., don’t tell a peer a route learned from other peer
–Manipulate attributes to control what they see
 E.g., make a path look artificially longer than it is
24
BGP Policy Examples
25
Import Policy: Local Preference
• Favor one path over another
–Override the influence of AS path length
–Apply local policies to prefer a path
• Example: prefer customer over peer
Local-pref = 90
Sprint
AT&T
Local-pref = 100
Tier-2
Tier-3
Yale
26
Import Policy: Filtering
• Discard some route announcements
–Detect configuration mistakes and attacks
• Examples on session to a customer
–Discard route if customer doesn’t own the prefix
–Discard route containing other large ISPs
Patriot
USLEC
Princeton
128.112.0.0/16
27
Export Policy: Filtering
• Discard some route announcements
–Limit propagation of routing information
• Examples
–Don’t announce routes from one peer to another
UUNET
AT&T
Sprint
28
Export Policy: Filtering
• Discard some route announcements
–Limit propagation of routing information
• Examples
–Don’t announce routes for networkmanagement hosts or the underlying routers
themselves
USLEC
network
operator
Princeton
29
Export Policy: Attribute Manipulation
• Modify attributes of the active route
–To influence the way other ASes behave
• Example: AS prepending
–Artificially inflate AS path length seen by others
–Convince some ASes to send traffic another way
Sprint
USLEC
Patriot
88 88
Princeton
128.112.0.0/16
88
30
BGP Policy Configuration
• Policy languages are vendor-specific
–Not part of the BGP protocol specification
–Different languages for Cisco, Juniper, etc.
• Still, all languages have some key features
–Policy as a list of clauses
–Each clause matches on route attributes
–… and discards or modifies the matching routes
• Configuration done by human operators
–Implementing the policies of their AS
–Biz relationships, traffic engineering, security, …31
BGP Inside an AS
32
An AS is Not a Single Node
• Multiple routers in an AS
–Need to distribute BGP information within the AS
–Internal BGP (iBGP) sessions between routers
AS1
eBGP
iBGP
AS2
33
Internal BGP and Local Preference
• Example
– Both routers prefer the path through AS 100 on the left
– … even though the right router learns an external path
AS 200
AS 100
AS 300
Local Pref = 90
Local Pref = 100
I-BGP
AS 256
34
Joining BGP and IGP Information
• Border Gateway Protocol (BGP)
–Announces reachability to external destinations
–Maps a destination prefix to an egress point
 128.112.0.0/16 reached via 192.0.2.1
• Interior Gateway Protocol (IGP)
–Used to compute paths within the AS
–Maps an egress point to an outgoing link
 192.0.2.1 reached via 10.1.1.1
10.1.1.1
192.0.2.135
An AS May Learn Many Routes
• Multiple connections to neighboring ASes
–Multiple border routers may learn good routes
–… with the same local-pref and AS path length
Multiple links
4
3
5
2
7
6
1
36
Hot-Potato (Early-Exit) Routing
• Hot-potato routing
– Each router selects the closest egress point
– … based on the path cost in intradomain protocol
• BGP decision process
– Highest local preference
– Shortest AS path
– Closest egress point
– Arbitrary tie break
dst
A
3
4
D
8 10
3
8
F 5
B
9
E
4
G
C
hot potato
37